The present invention relates generally to trip units. More specifically, the present invention relates to a method of heat index accumulator initialization for short time and groundfault protection algorithms in an electronic trip unit.
Electronic trip units are well known. Electronic trip units typically comprise voltage and current sensors which provide analog signals indicative between the power line signals. The analog signals are converted by an A/D (analog/digital) converter to digital signals which are processed by a microcontroller. The trip unit further includes RAM (random access memory), ROM (read only memory) and EEPROM (electronic erasable programmable read only memory) all of which interface with the microcontroller. The ROM includes trip unit application code, e.g., main functionality firmware, including initializing parameters, and boot code. The EEPROM includes operational parameters for the application code.
These trip units are required to meet certain standards, e.g., UL/ANSI/IEC, which specify a short time delay from the instant power is applied to when a trip unit must be ready to trip. In this context, "short time" delay refers to a specific protection characteristic, i.e. short time vs. long time, instantaneous, or ground fault protection. Although the delay is literally "short", the context of the statement is in reference to the short time protection characteristic curve. The standards also define trip time curves specifying under what conditions a trip must occur.
Short time and groundfault protection heat index accumulators are initialized to compensate for current flow during the time interval between the application of phase current and application of current measurement and protection algorithms. This time interval is assumed to be a predetermined amount of time. This assumed time delay is applied to the initial current measurement (or a derivative thereof) to determine an initial value (or a component thereof, for short time and/or groundfault heat index accumulators, as is well known in the art.